As a requirement of a wireless communications system for performance such as wide coverage and a high data rate increases, a phased-array transmitter featuring spatial power combination, beam steering, high system efficiency, high scanning resolution, and a low phase/amplitude error is urgently demanded.
The following two types of transmitters exist in the prior art:
1. A phased-array transmitter (phased-array transmitter) is a transmitter having multiple radio frequency outputs. Spatial power combination is performed for each radio frequency output by using an antenna. The phased-array transmitter implements antenna beam direction steering and scanning and the like by controlling a phase of each radio frequency output. As shown in FIG. 1, an existing phased-array transmitter 100 using an analog modulation technology mainly includes a balun 101, a 1:4 power splitter 102, a phase-shift circuit 103, a power amplifier (PA) 104, an output matching circuit 105, and a digital controller 106. An input signal PIN indicates a radio frequency signal obtained after analog modulation is performed on a baseband signal. PIN is divided into four signals after passing through the power splitter 102. After the phase-shift circuit 103 performs phase shifting and the PA 104 performs amplification on the four signals, four radio frequency signals (that is, Pout1, Pout2, Pout3, and Pout4) are output from the output matching circuit 105. The digital controller 106 is configured to control the phase-shift circuit 103 to perform phase shifting. The existing phased-array transmitter 100 has the following disadvantages: 1. Usually, system efficiency of the phased-array transmitter 100 is relatively low due to a need to balance system efficiency and linearity (to implement high linearity, the phased-array transmitter 100 cannot be used when an output power is greater than P1dB, where P1dB indicates an operating point at which a gain compression of output to input is equal to 1 dB, and dB indicates decibel). This further leads to power consumption. Therefore, the phased-array transmitter 100 is not suitable in a mobile device focusing on energy saving. 2. The phased-array transmitter 100 uses a radio frequency phase shift (RF phase shift) technology, and phase shifters in the phase-shift circuit 103 are connected in series in a radio frequency path. Non-linearity of the phase shifters may bring an amplitude error, and it is difficult for an architecture of the phased-array transmitter 100 to eliminate the amplitude error. Therefore, transmitter performance is severely affected.
2. A polar transmitter (polar transmitter) is also referred to as an envelope tracking (envelope tracking, ET) transmitter. The polar transmitter can convert a baseband signal into a phase signal and an amplitude signal, use the amplitude signal to control an output power of a PA, perform phase modulation on the phase signal, combine the phase signal obtained after the phase modulation and the amplitude signal into a radio frequency signal at a PA stage, and output the radio frequency signal. FIG. 2 shows an existing analog polar transmitter 200. A polar signal generator 201 coverts in-phase (in-phase, I)/quadrature (quadrature, Q) baseband signals into an amplitude signal A and a phase signal Φ. Digital-to-analog converters (Digital to Analog Converter, DAC) 202 and 203 respectively converts the phase signal Φ and the amplitude signal A in a digital domain into signals in an analog domain. Then an amplitude modulation circuit 206 performs modulation on an analog amplitude signal to obtain an amplitude modulation signal AM. A phase modulator 205 performs phase modulation on an analog phase signal Φ by using a local oscillator signal to obtain a phase modulation signal PM. Finally, a PA 204 combines the phase modulation signal PM and the amplitude modulation signal AM into a radio frequency signal. The analog polar transmitter 200 has disadvantages of a low data rate and narrow channel bandwidth.
Therefore, it is urgent to develop a new transmitter to meet a requirement of a wireless communications system for performance such as a wide frequency band, high scanning resolution, a low phase/amplitude error, and the like.